Negative electrode material for lithium-ion secondary battery, method for manufacturing negative electrode material for lithium-ion secondary battery, negative electrode material slurry for lithium-ion secondary battery, negative electrode for lithium-ion secondary battery, and lithium-ion secondary battery

ABSTRACT

A negative electrode material for a lithium-ion secondary battery, in which the negative electrode material includes a composite particle including a spherical graphite particle and plural graphite particles that have a compressed shape and that aggregate or are combined so as to have nonparallel orientation planes, and the negative electrode material has an R-value in a Raman measurement of from 0.03 to 0.10, and has a pore volume as obtained by mercury porosimetry of from 0.2 mL/g to 1.0 mL/g in a pore diameter range of from 0.1 μm to 8 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 15/127,875, filed inthe U.S on Sep. 21, 2016, which is a national phase application filedunder 35 U.S.C. § 371 of International Application No.PCT/JP2015/058980, filed on Mar. 24, 2015, which claims priority fromJapanese Patent Application No. 2014-062431, filed Mar. 25, 2014, theentire content of each of which are hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a negative electrode material for alithium-ion secondary battery, a method for manufacturing a negativeelectrode material for a lithium-ion secondary battery, a negativeelectrode material slurry for a lithium-ion secondary battery, anegative electrode for a lithium-ion secondary battery, and alithium-ion secondary battery.

BACKGROUND ART

Lithium-ion secondary batteries have a higher energy density compared toother secondary batteries such as nickel-cadmium batteries,nickel-hydrogen batteries, or lead storage batteries. Thus, lithium-ionsecondary batteries are used as power sources for portable electronicdevices such as notebook computers and mobile phones.

Recent trends in development of lithium-ion secondary batteries includedownsizing of batteries for resource saving and cost reduction, as wellas expansion of applications to electric vehicles and power sources forelectricity storage. Thus, there is a need to increase the density ofthe negative electrode for achieving a higher capacity, a higherinput-output efficiency, and cost reduction. High-crystallinity carbonmaterials, such as artificial graphite or spherical natural graphiteobtained by spheroidizing flake-shaped natural graphite, have drawnattention as a material for obtaining a high-density negative electrode.

In the case of artificial graphite, as described in Japanese PatentApplication Laid-Open (JP-A) No. H10-158005, cycling performance andrapid charge-discharge properties are improved by using a graphiteparticle, having a secondary particle structure in which plural primaryparticles having a compressed shape aggregate or are combined so as tohave nonparallel orientation planes, for a negative electrode material.

In a lithium-ion secondary battery, an energy density per volume can beincreased by increasing the negative electrode density as mentionedabove. However, the application of excessive pressure as high as 1.7g/cm³ or more to a negative electrode in order to increase the densitythereof may cause many problems, such as peeling of graphite in thenegative electrode from a current collector and deterioration ofcharge-discharge properties due to the high crystalline anisotropy ofgraphite.

Spherical natural graphite is characterized in that it has good peelstrength and thus is hardly peeled off from a current collector evenwhen an electrode is pressed with a strong force. However, sphericalnatural graphite has high reaction activity with an electrolyticsolution and low permeability to an electrolytic solution. Therefore,first cycle charge-discharge efficiency and rapid charge-dischargeefficiency are still scope for improvement.

SUMMARY OF INVENTION Technical Problem

In the negative electrode material using artificial graphite having thesecondary particle structure, a current collector is coated with thenegative electrode material and then pressed with high pressure toincrease the density. In this case, the primary particles that form thesecondary particle are oriented parallel to the current collector, whichmay in inhibition of lithium ion migration to a positive electrode anddeterioration of cycling performance. The pressing pressure after thecoating can be reduced by adding spherical natural graphite for thepurpose of increasing the density of the negative electrode materialitself. However, there is the problem that lattice defects present onthe surface of the spherical natural graphite easily react with anelectrolytic solution.

Spherical natural graphite coated with a low crystallinity carbon andthe like requires a strong pressing pressure for being hardened, andthus the desired density is not always achieved. Furthermore, a pressingtreatment to adjust the electrode density may cause peeling of a coatinglayer or a defect in a coating layer, which may result in deteriorationof charge-discharge properties, cycling performance, and safety.

In view of the facts above, it is an object of the present invention toprovide a negative electrode material for a lithium-ion secondarybattery that can provide a lithium-ion secondary battery having improvedhigh-load characteristics even when subjected to a treatment to increasethe electrode density; a method for manufacturing the negative electrodematerial for a lithium-ion secondary battery; a negative electrodematerial slurry for a lithium-ion secondary battery; a negativeelectrode for a lithium-ion secondary battery; and a lithium-ionsecondary battery.

Solution to Problem

As a result of intensive studies by the inventors, a negative electrodematerial for a lithium-ion secondary battery in which the negativeelectrode material includes a composite particle containing a sphericalgraphite particle and plural graphite particles that have a compressedshape and that aggregate or are combined so as to have nonparallelorientation planes, and the negative electrode material has an R-valuein a Raman measurement of from 0.03 to 0.10, and has a pore volume asobtained by mercury porosimetry of from 0.2 mL/g to 1.0 mL/g in a porediameter range of from 0.1 μm to 8 μm, is found to be effective toapproach the above problems and the present invention has beencompleted.

Specific means for solving the above problems include the followingembodiments.

<1>A negative electrode material for a lithium-ion secondary battery,the negative electrode material including a composite particle thatincludes a spherical graphite particle and a plurality of graphiteparticles that have a compressed shape and that aggregate or arecombined so as to have nonparallel orientation planes, and the negativeelectrode material having an R-value in a Raman measurement of from 0.03to 0.10, and having a pore volume as obtained by mercury porosimetry offrom 0.2 mL/g to 1.0 mL/g in a pore diameter range of from 0.1 μm to 8μm.

<2> The negative electrode material for a lithium-ion secondary batteryaccording to <1>, in which a specific surface area of the negativeelectrode material, as measured by a BET method, is from 1.5 m²/g to 6.0m²/g.

<3> The negative electrode material for a lithium-ion secondary batteryaccording to <1> or <2>, in which a saturated tap density of thenegative electrode material is from 0.8 g/cm³ to 1.2 g/cm³.

<4> The negative electrode material for a lithium-ion secondary batteryaccording to any one of <1> to <3>, in which the negative electrodematerial has an intensity ratio (P₂/P₁) of a diffraction peak (P₂) for a(101) plane of a rhombohedral crystal structure to a diffraction peak(P₁) for a (101) plane of a hexagonal crystal structure in a CuKα X-raydiffraction pattern is 0.35 or less.

<5> The negative electrode material for a lithium-ion secondary batteryaccording to any one of <1> to <4>, in which the spherical graphiteparticle has a circularity of 0.8 or higher.

<6>A method of manufacturing the negative electrode material for alithium-ion secondary battery according to any one of <1> to <5>,including steps of:

(a) mixing a graphitizable aggregate or graphite with a graphitizablebinder, a graphitization catalyst, and a spherical graphite particle;and

(b) calcining the mixture.

<7> The method of manufacturing the negative electrode material for alithium-ion secondary battery according to <6>, including, between thesteps (a) and (b), at least one step selected from the group consistingof (c) molding the mixture and (d) subjecting the mixture to a heattreatment.

<8>A negative electrode material slurry for a lithium-ion secondarybattery, the negative electrode material slurry including:

the negative electrode material for a lithium-ion secondary batteryaccording to any one of <1> to <5> or a negative electrode material fora lithium-ion secondary battery manufactured by the method ofmanufacturing a negative electrode material for a lithium-ion secondarybattery according to <6> or <7>;

an organic binder; and

a solvent.

<9>A negative electrode for a lithium-ion secondary battery, thenegative electrode including:

a current collector; and

a negative electrode material layer formed on the current collector andincluding the negative electrode material for a lithium-ion secondarybattery according to any one of <1> to <5>.

<10>A lithium-ion secondary battery, including:

a positive electrode;

an electrolyte; and

the negative electrode for a lithium-ion secondary battery according to<9>.

Advantageous Effects of Invention

According to the invention, there can be provided a negative electrodematerial for a lithium-ion secondary battery that can provide alithium-ion secondary battery having improved high-load characteristicseven when subjected to a treatment to increase the electrode density; amethod of manufacturing the negative electrode material for alithium-ion secondary battery; a negative electrode material slurry fora lithium-ion secondary battery; a negative electrode for a lithium-ionsecondary battery; and a lithium-ion secondary battery.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a view showing an example of a scanning electron micrographic(SEM) image of a composite particle included in a negative electrodematerial for a lithium-ion secondary battery according to the presentinvention.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, specific embodiments of the present invention are describedin detail. However, the invention is not limited to these embodiments.In the embodiments below, the components (including element steps andthe like) are not always indispensable unless otherwise expresslyprovided or except the case in which the components are apparentlyindispensable in principle. The same applies to numerical values andranges of the components, and the invention is not limited thereby.

The term “step” as used herein includes not only a separate step butalso a step that is not clearly distinguished from other steps as longas the desired effect of the step is obtained therefrom. As used herein,the notation “to” expressing a numerical range indicates a rangeincluding the numerical values before and after “to”, as the minimumvalue and the maximum value, respectively. As regard to the amount of acomponent of a composition, when plural substances corresponding to thesame component exist in the composition, the amount of the component inthe composition refers to a total amount of the plural substances in thecomposition unless otherwise specified. As regard to a particle size ofa component of a composition, when plural particles corresponding to thesame component exist in the composition, the particle size of thecomponent in the composition refers to a value obtained from the mixtureof the plural particles exist in the composition unless otherwisespecified. The term “layer” as used herein includes not only a structureconfigured to cover an entire surface but also a structure configured tocover a part of a surface when observed in planer view. The term“layered” refers to stacking of layers, where two or more layers may bebonded together or may be detachable from each other.

<Negative Electrode Material for Lithium-Ion Secondary Battery>

A negative electrode material for a lithium-ion secondary batteryaccording to the invention includes a composite particle that includes aspherical graphite particle and plural graphite particles that have acompressed shape and that aggregate or are combined so as to havenonparallel orientation planes, in which the negative electrode materialhas an R-value in a Raman measurement of from 0.03 to 0.10, and has apore volume as obtained by mercury porosimetry of from 0.2 mL/g to 1.0mL/g in a pore diameter range of from 0.1 μm to 8 μm.

In a case in which the negative electrode material for a lithium-ionsecondary battery is used, a lithium-ion secondary battery havingimproved high-load characteristics even when subjected to a treatment toincrease the electrode density may be obtained. In a case in which thenegative electrode material for a lithium-ion secondary batteryaccording to the invention is used, peeling of a negative electrodeactive material from a current collector may be suppressed even when thenegative electrode for a lithium-ion secondary battery is subjected to atreatment to increase the electrode density, and also the density of thenegative electrode may be easily increased. Furthermore, even in a casein which a high-density negative electrode is used, a lithium-ionsecondary battery with improved capacity, efficiency, liquid absorbingproperties, safety, low-temperature characteristics, charge-dischargeload characteristics, and cycle life may be obtained.

Composite Particle

The composite particle is not particularly limited as long as thecomposite particle includes a spherical graphite particle and pluralgraphite particles that have a compressed shape and that aggregate orare combined so as to have nonparallel orientation planes. For example,the composite particle may be one in which plural graphite particleshaving a compressed shape aggregate or are combined so as to havenonparallel orientation planes and, further, are combined with at leasta part of the surface of the spherical graphite particle. Morespecifically, the graphite particles having a compressed shape may becombined with at least a part of the surface of the spherical graphiteparticle via a carbonic substance derived from a binder. The formationof the composite particle can be confirmed by, for example, theobservation by a scanning electron microscope (SEM).

FIG. 1 is a view showing an example of the SEM image of the compositeparticle included in the negative electrode material for a lithium-ionsecondary battery according to the invention. The part indicated by thedotted line in FIG. 1 corresponds to a spherical graphite particle. Thecomposite particle (the part indicated by the solid line in FIG. 1) isformed by the spherical graphite particle and plural graphite particlespresent around the spherical graphite particle, in which the graphiteparticles have a compressed shape and that aggregate or are combined soas to have nonparallel orientation planes.

The negative electrode material for a lithium-ion secondary battery mayinclude, in addition to the composite particle, a graphite particlehaving a compressed shape that does not form a composite particle, aspherical graphite particle, or an aggregate graphite particle formed byaggregating or combining plural of the graphite particles having acompressed shape.

Average Particle Size (Median Diameter)

The average particle size (median diameter) of the negative electrodematerial for a lithium-ion secondary battery is not particularlylimited, and may be from 10 μm to 30 μm or from 15 μm to 25 μm, in termsof the influence on orientation and permeability to an electrolyticsolution. The average particle size can be measured using a laserdiffraction particle size distribution analyzer, and means a particlesize (D50) at which a cumulative volume from the small diameter side ofthe obtained particle size distribution in terms of mass reaches 50%.The average particle size of the negative electrode material for alithium-ion secondary battery means an average value of particlesincluding the composite particles and graphite particles that do notform the composite particles.

The average particle size can be measured using a laser diffractionparticle size distribution analyzer (SALD-3000J, manufactured byShimadzu Corporation) under the following conditions.

Absorbance: 0.05 to 0.20

Sonication: 1 to 3 minutes

Examples of the method of measuring the average particle size in thecase of using the negative electrode material for a lithium-ionsecondary battery as a negative electrode include a method in which anelectrode sample or an electrode to be observed is embedded in an epoxyresin and subjected to a mirror-polishing process, and then theelectrode section is observed by a scanning electron microscope; and amethod in which an electrode section prepared using an ion millingapparatus (E-3500, manufactured by Hitachi High-TechnologiesCorporation) is observed by a scanning electron microscope. The averageparticle size in this case is a median value of particle sizes of 100particles randomly selected from the composite particles and thegraphite particles that do not form the composite particles.

The electrode sample can be prepared, for example, by adding water to,as a solid component, a mixture of 98 parts by mass of the negativeelectrode material for a lithium-ion secondary battery, 1 part by massof a styrene butadiene resin as a binder, and 1 part by mass ofcarboxymethyl cellulose as a thickener such that the mixture has aviscosity of from 1500 mPa·s to 2500 mPa·s at 25° C. to prepare adispersion, coating a copper foil having a thickness of 10 μm with thedispersion so as to give a thickness of about 70 μm (when applied), andthen dried the resultant at 120° C. for 1 hour.

Graphite Particles that have a Compressed Shape

The composite particle includes plural graphite particles that have acompressed shape and that aggregate or are combined so as to havenonparallel orientation planes.

The graphite particle having a compressed shape has a non-sphericalshape having a major axis and a minor axis. Examples thereof include agraphite particle having a scaly shape, a graphite particle having aflake shape, and a graphite particle having a partially clumpy shape.More specifically, when a length along the major axis is defined as Aand a length along the minor axis is defined as B, the aspect ratiorepresented by A/B may be from 1.2 to 5, or may be from 1.3 to 3. Theaspect ratio is obtained by enlarging the graphite particles by amicroscope, measuring respective A/B of randomly selected 100 graphiteparticles, and calculating an average value of the measured values.

The condition in which the graphite particles having a compressed shapehave nonparallel orientation planes means a condition in which eachplane (orientation plane) parallel to a plane having a maximum sectionalarea of two or more of the graphite particles having a compressed shapeis not in mutually parallel relationship to the others. The conditionwhether or not the graphite particles having a compressed shape havemutually nonparallel orientation planes can be confirmed by themicrographic observation. In a case in which the graphite particleshaving a compressed shape aggregate or are combined so as to havemutually nonparallel orientation planes, advantageous effects, such assuppression of an increase in the degree of orientation of the particleson the electrode and achievement of high charge-discharge capacity, maybe obtained.

The condition in which the graphite particles having a compressed shapeaggregate or are combined means a condition in which two or moregraphite particles having a compressed shape aggregate or are combinedchemically via a carbonic substance. The carbonic substance may be acarbonic substance that is a carbonized product of a binder such as taror pitch obtained by a calcination process. In terms of mechanicalstrength, the particles may be combined. The condition whether or notthe graphite particles having a compressed shape aggregate or arecombined can be confirmed by, for example, the observation by a scanningelectron microscope.

The number of the graphite particles that have a compressed shape andthat aggregate or are combined may be 3 or more, or may be 10 or more.

Regarding the size of an individual graphite particle having acompressed shape, the average particle size D50 may be 50 μm or less, ormay be 25 μm or less, from the viewpoint that the particles can easilyaggregate or are combined. The average particle size D50 may be 1 μm ormore. The average particle size D50 can be measured by a laserdiffraction particle size distribution analyzer, and means a particlesize at which a cumulative volume from the small diameter side of theobtained particle size distribution in terms of mass reaches 50%.

The raw material of the graphite particle having a compressed shape isnot particularly limited, and examples thereof include artificialgraphite, natural graphite, coke, a resin, tar, and pitch. Among these,graphite obtained from artificial graphite, natural graphite, or cokecan form a soft particle having a high crystallinity and, as a result ofwhich the density of the electrode formed therefrom tends to be easilyincreased. In addition, in a case in which graphite having a highcrystallinity is used, the R-value of the negative electrode materialfor a lithium-ion secondary battery tends to be reduced and first cyclecharge-discharge efficiency tends to be improved.

Spherical Graphite Particle

The composite particle includes a spherical graphite particle. In a casein which the spherical graphite particle having a high density isincreased, the density of the negative electrode material may beincreased as compared to a case in which only the graphite particleshaving a compressed shape are included, and the pressure to be appliedin the treatment to increase the electrode density may be reduced. As aresult, a phenomenon in which the graphite particles having a compressedshape are oriented in the direction parallel to the current collectorand prevent movement of lithium ions may be suppressed.

Examples of the spherical graphite particle and a raw material thereofinclude spherical artificial graphite and spherical natural graphite. Interms of achievement of a sufficient saturated tap density of thenegative electrode material, the spherical graphite particle may be agraphite particle having a high density. Specifically, the sphericalgraphite particle may be spherical natural graphite that has beensubjected to a spheroidizing treatment to increase the tap density, ormay be a spherical graphite particle that has been calcined at 1500° C.or higher. In a case in which a spherical graphite particle used as araw material is subjected to a calcination treatment at 1500° C. orhigher, a spherical graphite particle having a high crystallinity may beobtained, and the R-value of the negative electrode material for alithium-ion secondary battery may be reduced as described above.

The average particle size of the spherical graphite particle is notparticularly limited, and may be from 5 μm to 40 μm, from 8 μm to 35 μm,or from 10 μm to 30 μm. The average particle size thereof can bemeasured by a laser diffraction particle size distribution analyzer, andmeans a particle size at which a cumulative volume from the smalldiameter side of the obtained particle size distribution in terms ofmass reaches 50%.

Circularity of Spherical Graphite Particle

The circularity of the spherical graphite particle may be 0.80 orhigher, or 0.85 or higher. Some spherical graphite particles aredeformed by a mechanical force during the manufacture of the negativeelectrode material for a lithium-ion secondary battery. However, thedegree of orientation in the negative electrode material may be reducedand characteristics of the electrode may be improved as the overallcircularity of the spherical graphite particles included in the negativeelectrode material for a lithium-ion secondary battery is increased.Examples of methods for increasing the circularity of the sphericalgraphite particle included in the negative electrode material for alithium-ion secondary battery include use of a spherical graphiteparticle having high circularity as a raw material. The circularity isobtained by measuring a portion of the spherical graphite particleincluded in the composite particle.

The circularity of the spherical graphite particle can be determined byphotographing a section of the spherical graphite particle andcalculating according to the following Equation:Circularity=(a perimeter of an equivalent circle)/(a perimeter of thesectional image of a spherical graphite particle)

Herein, the “equivalent circle” is a circle having the same area as thesectional image of the spherical graphite particle. The perimeter of thesection image of the spherical graphite particle is the length of theoutline of the sectional image of the photographed spherical graphiteparticle. The circularity in the invention represents a value obtainedby observing the sectional image of the spherical graphite particleenlarged 1000-fold using a scanning electron microscope, selectingrandomly 10 spherical graphite particles, measuring the circularity ofeach of the spherical graphite particles according to the above method,and calculating a mean value.

Examples of the method for measuring the circularity of the sphericalgraphite particle using the negative electrode material for alithium-ion secondary battery in a negative electrode include a methodin which an electrode sample or an electrode to be observed is embeddedin an epoxy resin and subjected to a mirror-polishing process, and thenthe electrode section is observed by a scanning electron microscope; anda method in which an electrode section prepared using an ion millingapparatus (E-3500, manufactured by Hitachi High-TechnologiesCorporation) is observed by a scanning electron microscope.

The sample electrode can be produced, for example, in the same manner asthe sample electrode used for measuring the average particle sizedescribed above.

R-value in Raman Measurement

The negative electrode material for a lithium-ion secondary battery hasan R-value in a Raman measurement of from 0.03 to 0.10. The R-value maybe from 0.04 to 0.10, or may be from 0.05 to 0.10. In a case in whichthe R-value exceeds 0.10, gaseous swelling in a lithium-ion secondarybattery due to an increased amount of a decomposed product of anelectrolytic solution may occur, or first cycle efficiency may bereduced. As a result of which, it may be substantially difficult toapply the negative electrode material to a high density-adaptiveelectrode. In a case in which the R-value is less than 0.03, the numberof lattice defects in graphite for intercalation and de-intercalation ofa lithium ion is too small, and thus charge-discharge loadcharacteristics may be deteriorated.

The R-value is defined as an intensity ratio (1B/IA) of a maximum peakintensity IB near 1360 cm⁻¹ to a maximum peak intensity IA near 1580cm⁻¹ in a Raman spectrum obtained in a Raman measurement describedbelow.

The Raman measurement is performed using a Raman spectrometer “a laserRaman spectrometer” (model number: NRS-1000, manufactured by JASCOCorporation), by irradiating with argon-ion laser a sample plate onwhich the negative electrode material for a lithium-ion secondarybattery or an electrode obtained by applying the negative electrodematerial for a lithium-ion secondary battery on a current collector andpressing the resultant is set to be flat. Measurement conditions are asfollows:

Wavelength of argon laser light: 532 nm

Wavenumber resolution: 2.56 cm⁻¹

Measurement range: 1180 cm⁻¹ to 1730 cm⁻¹

Peak Research: Background Removal

Examples of the method of obtaining the negative electrode material fora lithium-ion secondary battery that has an R-value in the Ramanmeasurement of from 0.03 to 0.10 include a method of calcining thespherical graphite particle as described above. Examples thereof furtherincludes a method of adjusting the percentage of the carbon residue,which is derived from a binder component such as pitch used as a rawmaterial, to 30% by mass or less with respect to the total mass of thenegative electrode material for a lithium-ion secondary battery. While acomponent having a low crystallinity, such as the binder component, isnecessary for forming the composite particle by aggregating or combiningthe above-described graphite particles having a compressed shape, such acomponent exhibits poor crystal growth as well as low residual carbonratio after a graphitization treatment. This leads to low productivity,and results in the formation of hard particles after a graphitizationtreatment. As a result, pressure is applied to the surface of thegraphite particle during a pulverizing process for the purpose ofadjusting the particle size or during pressing for adjusting the densityof the formed electrode, and lattice defects may be generated and theR-value may be increased. Accordingly, it is effective to limit theadditive amount of the binder component such that the content of thecarbon residue of the binder component is limited to 30% by mass or lesswith respect to the total mass of the negative electrode material for alithium-ion secondary battery.

Pore Volume

The negative electrode material for a lithium-ion secondary battery hasa pore volume as obtained by mercury porosimetry of from 0.2 mL/g to 1.0mL/g in a pore diameter range of from 0.1 μm to 8 μm. In a case in whichthe pore volume is less than 0.2 mL/g, the amount of the electrolyticsolution serving as a medium that allows movement of lithium ions is toosmall in a produced lithium-ion secondary battery, as a result of whichrapid charge-discharge properties tends to be deteriorated. On the otherhand, in a case in which the pore volume exceeds 1.0 mL/gm, oilabsorbing ability of an additive such an organic adhesive or a thickeneris increased, as a result of which productivity tends to be decreaseddue to abnormalities in paste viscosity and insufficient adhesion to thecurrent collector.

The pore volume as obtained by mercury porosimetry in a range of from0.1 μm to 8 μm may be from 0.4 mL/g to 0.8 mL/g, or may be from 0.5 mL/gto 0.7 mL/g. The pore volume of the negative electrode material for alithium-ion secondary battery can be set in the above range, forexample, by appropriately adjusting a blending ratio of the sphericalgraphite particle.

The pore volume can be obtained by mercury porosimetry as describedbelow.

In the mercury porosimetry, “a porosimetry analyzer AUTOPORE 9520”manufactured by Shimadzu Corporation is used. The mercury parameters areset to a mercury contact angle of 130.0° and a mercury surface tensionof 485.0 mN/m (485.0 dynes/cm). A sample (about 0.3 g) is placed into astandard cell and measured under a condition of an initial pressure of 9kPa (corresponding to about 1.3 psia and a pore diameter of about 140μm). The capacity of a pore volume in a range of from 0.1 to 8 μm iscalculated based on the obtained pore distribution.

Specific Surface Area

The specific surface area of the negative electrode material for alithium-ion secondary battery, as measured by a BET method, may be from1.5 m²/g to 6.0 m²/g, or may be from 2.5 m²/g to 5.0 m²/g.

The specific surface area is an indicator of an area of an interfacewith an electrolytic solution. Specifically, in a case in which thevalue of the specific surface area of the negative electrode material is6.0 m²/g or less, the area of the interface between the negativeelectrode material for a lithium-ion secondary battery and theelectrolytic solution is not too large, as a result of which gasgeneration may be suppressed due to the suppression of increase inreaction field for the decomposition reaction of the electrolyticsolution and first cycle charge-discharge efficiency may be improved. Ina case in which the value of the specific surface area of the negativeelectrode material is 1.5 m²/g or more, load is suppressed since acurrent density applied per unit area does not rapidly increase, as aresult of which charge-discharge efficiency, charge acceptability, rapidcharge-discharge properties, and the like tend to be improved.

The specific surface area of the negative electrode material can bemeasured in accordance with a known method such as a BET method (anitrogen gas adsorption method). Preferably, the negative electrodematerial for a lithium-ion secondary battery or an electrode obtained byapplying the negative electrode material for a lithium-ion secondarybattery to a current collector and pressing the resultant is mounted ina measuring cell and subjected to a preheating treatment at 200° C.while vacuum degassing to obtain a sample, and then nitrogen gas isallowed to adsorb onto the sample using a gas adsorption apparatus (ASAP2010, manufactured by Shimadzu Corporation). The obtained sample issubjected to a BET analysis using a five point method to calculate aspecific surface area. The specific surface area of the negativeelectrode material for a lithium-ion secondary battery can be set in therange described above, for example, by adjusting the average particlesize (i.e., the specific surface area tends to be increased as theaverage particle size reduces, whereas the specific surface area tendsto be reduced as the average particle size increases).

Saturated Tap Density

The saturated tap density of the negative electrode material for alithium-ion secondary battery may be from 0.8 g/cm³ to 1.2 g/cm³, or maybe from 0.9 g/cm³ to 1.1 g/cm³.

The saturated tap density is an indicator of electrode densification. Ina case in which the saturated tap density of the negative electrodematerial is 1.2 g/cm³ or less, the density of the electrode obtained bycoating the current collector with the negative electrode material for alithium-ion secondary battery is high, as a result of which a pressureto be applied for adjusting the electrode density may be reduced and theoriginal shape of the graphite particle in the electrode may be easilymaintained. In a case in which the original shape of the graphiteparticles is maintain, it is advantageous in that the orientation of anelectrode plate is reduced and lithium ions easily move in and out,which may result in improvement of cycling performance. In a case inwhich the saturated tap density of the negative electrode material istoo high, the pore volume is reduced, and in a produced cell, the amountof the electrolytic solution serving as a medium that allows themovement of lithium ions is reduced, which may result in deteriorationof rapid charge-discharge properties. Accordingly, it is preferable toadjust the saturated tap density of the negative electrode material suchthat the pore volume is not too low. The saturated tap density of thenegative electrode material can be set in the above range byappropriately adjusting the percentage of the spherical graphiteparticle (i.e., the tap density tends to be increased as the ratio ofthe spherical graphite particle increases, whereas the tap density tendsto be reduced as the ratio of the spherical graphite particle reduces).

The saturated tap density can be measured in accordance with a knownmethod. Preferably, 100 ml of the negative electrode material for alithium-ion secondary battery is placed in a graduated cylinder andtapped (allowed the graduated cylinder to drop from a predeterminedheight) until the density reaches saturation using a tap densitymeasurement apparatus (KRS-406 manufactured by Kuramochi ScientificInstruments Co., Ltd), and then the saturated tap density is calculated.

Peak Intensity Ratio for Rhombohedral Structure

The negative electrode material for a lithium-ion secondary battery mayhave an intensity ratio (P₂/P₁) of a diffraction peak (P₂) for a (101)plane of a rhombohedral crystal structure to a diffraction peak (P₁) fora (101) plane of a hexagonal crystal structure in a CuKα X-raydiffraction pattern of 0.35 or less, or 0.30 or less. In a case in whichthe peak intensity ratio (P₂/P₁) is 0.35 or less, the negative electrodematerial for a lithium-ion secondary battery tends to exhibit a higherdegree of graphitization, and the charge-discharge capacity tends to beincreased.

The peak intensity ratio for the rhombohedral structure can becalculated based on an intensity ratio between a diffraction line forthe rhombohedral structure (P1: a diffraction angle of 43.2°) and adiffraction line for the hexagonal crystal structure (P2: a diffractionangle of 44.3°) in the CuKα X-ray diffraction pattern. Herein, thediffraction angle is represented by 2θ (θ represents Bragg angle). Thediffraction line from the (101) plane of the rhombohedral structureappears at the diffraction angle of 43.2°, and the diffraction line fromthe (101) plane from the hexagonal crystal structure appears at thediffraction angle of 44.3°.

The peak intensity ratio for the rhombohedral structure can be set inthe above range by adjusting the degree of graphitization (for example,by adjusting a heat treatment temperature).

<Method of Manufacturing Negative Electrode Material for Lithium-IonSecondary Battery>

The method of manufacturing the negative electrode material for alithium-ion secondary battery includes the steps of: (a) mixing agraphitizable aggregate or graphite with a graphitizable binder, agraphitization catalyst, and a spherical graphite particle and (b)calcining the mixture.

According to the above method, there can be manufactured a negativeelectrode material for a lithium-ion secondary battery, in which thenegative electrode material includes a composite particle including aspherical graphite particle and plural graphite particles that have acompressed shape and that aggregate or are combined so as to havenonparallel orientation planes, and the negative electrode material hasan R-value in a Raman measurement of from 0.03 to 0.1, and has a porevolume as obtained by mercury porosimetry of from 0.2 mL/g to 1.0 mL/gin a pore diameter range of from 0.1 μm to 8 μm.

Furthermore, according to the above method, heavy metal, magneticforeign matter, and impurities contained in the raw material are removedat high temperature when the raw material is graphitized by calcination.Therefore, an acid treatment, washing with water, or the like is notrequired for the spherical graphite particle such as natural graphite.As a result of which production cost may be reduced and a highly safenegative electrode material for a lithium-ion secondary battery may beprovided. Furthermore, in a case in which the spherical graphiteparticle that has been already graphitized is used as at least a part ofthe raw materials together with the graphitizable aggregate, productioncost may be reduced due to reduction in amount of a graphitizationcatalyst required for graphitizing the raw materials, reduction incalcination time for the graphitization, and the like. As a result ofwhich the negative electrode material for a lithium-ion secondarybattery may be provided at a low price while using expensive artificialgraphite. In addition, the amount of the binder component to be used formanufacturing the negative electrode material for a lithium-ionsecondary battery may be reduced.

In the above method, the spherical graphite particle is also calcinedtogether with other raw materials. As a result, the R-value in the Ramanmeasurement of the negative electrode material for a lithium-ionsecondary cell may be lowered as compared to a case in which thespherical graphite particle is mixed with a graphitized materialobtained by the calcination of other raw material.

In the step (a), a graphitizable aggregate or graphite is mixed with agraphitizable binder, a graphitization catalyst, and a sphericalgraphite particle, thereby obtaining a mixture.

Examples of the graphitizable aggregate include coke such as fluid coke,needle coke, and mosaic coke. In addition, an aggregate that has beenalready graphitized such as natural graphite or artificial graphite maybe used. The graphitizable aggregate is not particularly limited as longas the aggregate is in powder form. Among these, the graphitizableaggregate may be an easily graphitizable coke powder such as needlecoke. The graphite is not particularly limited as long as the graphiteis in powder form. The particle size of the graphitizable aggregate orthe graphite is preferably less than the particle size of the graphiteparticle having a compressed shape.

Examples of the spherical graphite particle include spherical artificialgraphite particles and spherical natural graphite particles.

Examples of the graphitizable binder include coal-based,petroleum-based, or artificial pitches and tars, thermoplastic resins,and thermosetting resins.

Examples of the graphitization catalyst include substances having agraphitization catalytic effect such as silicon, iron, nickel, titanium,or boron, and carbides thereof, oxides thereof, and nitrides thereof.

The content of the spherical graphite particle may be from 5% by mass to80% by mass, may be from 8% by mass to 75% by mass, or may be from 8% bymass to 70% by mass, with respect to 100 parts by mass of thegraphitizable aggregate or the graphite. In a case in which the contentof the spherical graphite particle is in the above range, a high densityand a high charge-discharge capacity tends to be obtained

The content of the graphitizable binder may be from 5% by mass to 80% bymass, may be 10% by mass to 80% by mass, or may be 15% by mass to 80% bymass, with respect to 100 parts by mass of the graphitizable aggregateor the graphite. By setting the additive amount of the graphitizablebinder to an appropriate amount, excess increase in the aspect ratio andspecific surface area of the graphite particle having a compressed shapeto be manufactured may be suppressed. Furthermore, excess increase inthe R-value in the Raman measurement may be suppressed by restrictingthe amount of the graphitizable binder such that the content of thecarbon residue derived from the binder after the calcination is 30% bymass or less with respect to the total mass of the negative electrodematerial for a lithium-ion secondary battery.

The graphitization catalyst may be added in an amount of from 1 part bymass to 50 parts by mass with respect to 100 parts by mass of the totalamount of the graphitizable aggregate or the graphite and thegraphitizable binder. In a case in which the amount of thegraphitization catalyst is 1 part by mass or more, the growth of crystalin the graphitic particle tends to be improved and charge-dischargecapacity tends to be improved. On the other hand, in a case in which theamount of the graphitization catalyst is 50 parts by mass or less, thegraphitizable aggregate or the graphite, the graphitizable binder, thegraphitization catalyst, and the spherical graphite particle may beeasily mixed homogeneously and operability tends to be improved. Themethod of mixing the graphitization catalyst is not particularly limitedas long as the method is a mixing method in which the graphitizationcatalyst is located on the inside of the particles or on the surface ofthe particles in the mixture at least before the calcination treatmentfor graphitization.

The method of mixing the graphitizable aggregate or the graphite withthe graphitizable binder, the graphitization catalyst, and the sphericalgraphite particle is not particularly limited. For example, the mixingmay be performed using a kneader or the like. The mixing may beperformed at a temperature equal to or higher than the softeningtemperature of the binder. Specifically, in a case in which thegraphitizable binder is pitch or tar, the temperature for the mixing maybe from 50° C. to 300° C., and in a case in which the binder is athermosetting resin, the temperature may be from 20° C. to 100° C.

In the step (b), the mixture obtained in the step (a) is calcined. Inthis step, the graphitizable component in the mixture is graphitized.The calcination is preferably conducted under an atmosphere in which themixture is hardly oxidized. Examples of the calcination method includecalcination in a nitrogen atmosphere, calcination in an argon gasatmosphere, or calcination in vacuum. The calcination temperature is notparticularly limited as long as the graphitizable component can begraphitized. For example, the calcination temperature may be 1500° C. orhigher, may be 2000° C. or higher, may be 2500° C. or higher, or may be2800° C. or higher. The calcination temperature may be 3200° C. orlower. In a case in which the calcination temperature is 1500° C. orhigher, a crystalline change occurs. In a case in which the calcinationtemperature is 2000° C. or higher, a graphite crystal grows well, andthe amount of the graphitization catalyst remaining in the producedgraphitic particle tends to be reduced (i.e., increase in an ash amountis suppressed). In any case, charge-discharge capacity and batterycycling performance tend to be improved. On the other hand, in a case inwhich the calcination temperature is 3200° C. or lower, sublimation of apart of the graphite may be suppressed.

The method of manufacturing the negative electrode material for alithium-ion secondary battery may include, between the steps (a) and(b), at least one step selected from the group consisting of (c) moldingthe mixture and (d) subjecting the mixture to a heat treatment.Specifically, only the step (b) may be performed after the step (a),only the step (c) may be performed after the step (a), the step (b) andthe step (c) may be performed in this order after the step (a), or thestep (c) and the step (b) may be performed in this order after the step(a).

Molding in the step (c) of molding the mixture can be performed, forexample, by pulverizing the mixture and placing the pulverized mixturein a container such as a mold.

Subjecting the mixture to the heat treatment in the step (d) ofsubjecting the mixture to the heat treatment is preferable, from theviewpoint of promoting graphitization. When performing the heattreatment, it is more preferable to conduct the heat treatment aftermolding the mixture in the step (c). The heat treatment may be conductedat 1500° C. or higher, or may be conducted at 2500° C. or higher.

In a case in which the particle size is not adjusted by molding andpulverizing the mixture before the calcination, the graphitized productobtained after the calcination may be pulverized into an intendedaverage particle size. Alternatively, the mixture may be molded andpulverized to adjust the particle size before the calcination, and thenthe obtained graphitized product may be further pulverized after thecalcination. The method for pulverizing the graphitized product is notparticularly limited. For example, the pulverization can be performed bya known method using a jet mill, a vibration mill, a pin mill, a hammermill, or the like. The average particle size (a median diameter) afterthe pulverization may be 100 μm or less, or may be from 10 μm to 50 μm.

The graphitized product after the calcination and pulverization may besubjected to an isotropic pressing treatment. Examples of the method ofthe isotropic pressing treatment include a method in which thegraphitized product obtained by the calcination and pulverization isplaced in a container made of rubber or the like and the container issealed, followed by subjecting the container an isotropic pressingtreatment using a pressing machine. The graphitized product that hasbeen subjected to the isotropic pressing treatment is preferably crushedby a cutter mill or the like and then graded with a sieve or the like.

The method described above is one example of the method of manufacturingthe negative electrode material for a lithium-ion secondary battery. Thenegative electrode material for a lithium-ion secondary battery may bemanufactured by any method other than the above-described method.Examples of the other method include a method in which graphiteparticles (i.e., aggregate graphite particles) are formed by aggregatingor combining plural graphite particles having a compressed shape so asto have nonparallel orientation planes, and then a spherical graphiteparticle is mixed therewith to form a composite particle. The method offorming the aggregate graphite particles may be referred to, forexample, descriptions of Japanese Patent No. 3285520 and Japanese PatentNo. 3325021.

Negative Electrode Active Material for Lithium-Ion Secondary Battery

The negative electrode active material for a lithium-ion secondarybattery according to the invention includes a carbonaceous particle or ametal particle for adsorption, which is different from the graphiteparticle included in the negative electrode for a lithium-ion secondarybattery in at least one of the shape or the physical properties. Thenegative electrode active material for a lithium-ion secondary batterypreferably further includes at least one structure that allowsadsorption of lithium ions selected from the group consisting of naturalgraphite, artificial graphite, amorphous coated graphite, resin coatedgraphite, amorphous carbon, and a metal particle for adsorption.

Negative Electrode Material Slurry for Lithium-Ion Secondary Battery

The negative electrode material slurry for a lithium-ion secondarybattery according to the invention includes the negative electrodematerial for a lithium-ion secondary battery or a negative electrodematerial for a lithium-ion secondary battery manufactured by the methodof manufacturing a negative electrode material for a lithium-ionsecondary battery, an organic binder, and a solvent.

The organic binder is not particularly limited. Examples of the organicbinder include polymer compounds such as styrene-butadiene rubbers;(meth)acrylic copolymers derived from an ethylenically unsaturatedcarboxylic acid ester (such as methyl(meth)acrylate,ethyl(meth)acrylate, butyl(meth)acrylate, (meth)acrylonitrile, orhydroxyethyl(meth)acrylate) and an ethylenically unsaturated carboxylicacid (such as acrylic acid, methacrylic acid, itaconic acid, fumaricacid, or maleic acid); polyvinylidene fluoride, polyethylene oxide,polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, andpolyamideimide.

The solvent is not particularly limited. Examples of the solvent to beused include organic solvents such as N-methylpyrrolidone,dimethylacetamide, dimethylformamide, and γ-butyrolactone.

The negative electrode material slurry for a lithium-ion secondarybattery may include a thickener for adjusting viscosity, if necessary.Examples of the thickener include carboxymethylcellulose,methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinylalcohol, polyacrylic acids and salts thereof, oxidized starch,phosphorylated starch, and casein.

An electroconductive auxiliary agent may be added to the negativeelectrode material slurry for a lithium-ion secondary battery, ifnecessary. Examples of the electroconductive auxiliary agent includecarbon black, graphite, acetylene black, an oxide having electricalconductivity, and nitride having electrical conductivity.

Negative Electrode for Lithium-Ion Secondary Battery

The negative electrode for a lithium-ion secondary battery according tothe invention includes a current collector, and a negative electrodematerial layer that is formed on the current collector and includes thenegative electrode material for a lithium-ion secondary battery.

The material and shape of the current collector are not particularlylimited. For example, a material made from a metal such as aluminum,copper, nickel, titanium, or stainless steel, or an alloy thereof, whichis formed into a belt form, such as a foil form, a perforated foil form,or a mesh belt form, may be used. In addition, a porous material such asa porous metal (foamed metal) or carbon paper may be used.

The method for forming the negative electrode material layer includingthe negative electrode material for a lithium-ion secondary battery onthe current collector is not particularly limited. Examples thereofinclude known methods such as a metal mask printing method, anelectrostatic coating method, a dip coating method, a spray coatingmethod, a roll coating method, a doctor blading method, a gravurecoating method, and a screen printing method. In a case in the negativeelectrode material layer is integrated with the current collector, theintegration may be carried out by a known method such as rolling,pressing, or a combination thereof.

The negative electrode for a lithium-ion secondary battery obtained byforming the negative electrode material layer on the current collectormay be subjected to a heat treatment depending on the kind of theorganic binder used. The heat treatment results in the removal of thesolvent and the curing of the binder and the strength of the negativeelectrode is highly intensified, whereby the adhesion between particlesand adhesion between the particles and the current collector may beimproved. The heat treatment may be carried out in an inert atmosphere,such as helium, argon, nitrogen, or in a vacuum atmosphere, in order toprevent oxidation of the current collector during the treatment.

The negative electrode for a lithium-ion secondary battery may bepressed (pressing treatment) before the heat treatment. By the pressingtreatment, the electrode density can be controlled. The electrodedensity may be from 1.5 g/cm³ to 1.9 g/cm³, or may be from 1.6 g/cm³ to1.8 g/cm³. As the electrode density is increased, the volume capacitytends to be increased, the adhesion of the negative electrode materiallayer to the current collector tends to be improved, and cyclingperformance tends to be improved.

Lithium-Ion Secondary Battery

A lithium-ion secondary battery according to the invention includes apositive electrode, an electrolyte, and the negative electrode for alithium-ion secondary battery. For example, the lithium-ion secondarybattery may have a configuration in which the negative electrode and thepositive electrode are arranged so as to be faced each other across aseparator, and in which an electrolytic solution containing anelectrolyte is injecting therein.

The positive electrode may be obtained similarly as the negativeelectrode by forming a positive electrode material layer on a currentcollector surface. In this case, as a current collector, a material madefrom a metal such as aluminum, titanium, or stainless steel, or an alloythereof, which is formed into a belt form, such as a foil form, aperforated foil form, or a mesh form, may be used.

A positive electrode material to be used in the positive electrode layeris not particularly limited. Examples thereof include a metalliccompound, a metallic oxide, a metallic sulfide, or an electricalconductive polymer material, which allows doping or intercalation oflithium ions. Furthermore, lithium cobaltate (LiCoO₂), lithium nickelate(LiNiO₂), lithium manganate (LiMnO₂), a double oxide thereof(LiCo_(x)Ni_(y)Mn_(z)O₂, x+y+z=1, 0<x, 0<y; LiNi_(2-x)Mn_(z)O₄, 0<x≤2),lithium manganese spinel (LiMn₂O₄), lithium vanadium compounds, V₂O₅,V₆O₁₃, VO₂, MnO₂, TiO₂, MoV₂O₈, TiS₂, V₂S₅, VS₂, MoS₂, MoS₃, Cr₃O₈,Cr₂O, olivine-type LiMPO₄ (M: Co, Ni, Mn, or Fe), an electricalconductive polymer such as polyacetylene, polyaniline, polypyrrole,polythiophene, or polyacene, or porous carbon may be used singly or incombination of two or more kinds thereof. Among these materials, lithiumnickelate (LiNiO₂) and a double oxide thereof (LiCo_(x)Ni_(y)Mn_(z)O₂,x+y+z=1, 0<x, 0<y; LiNi_(2-x)Mn_(z)O₄, 0<x≤2) have high capacity andthus suitable for the positive electrode material.

Examples of the separator include nonwoven fabric, cloth, a microporousfilm, and a combination thereof using as the main component a polyolefinsuch as polyethylene or polypropylene. In a case in which thelithium-ion secondary battery to be produced has a structure in which apositive electrode and a negative electrode do not contact directly, itis unnecessary to use a separator.

Examples of the electrolytic solution to be used include a so-calledorganic electrolytic solution in which a lithium salt such as LiClO₄,LiPF₆, LiAsF₆, LiBF₄, or LiSO₃CF₃ is dissolved in a non-aqueous solventcomposed singly or in a combination of two or more of ethylenecarbonate, propylene carbonate, butylene carbonate, vinylene carbonate,fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane,2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-on, γ-butyrolactone,dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butylethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane,tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate,or ethyl acetate. Among these, an electrolytic solution containingfluoroethylene carbonate is favorable, because a stable SEI (solidelectrolyte interface) tends to be formed therewith on the surface of anegative electrode material and cycling performance is significantlyimproved.

The structure of the lithium-ion secondary battery according to theinvention is not particularly limited, and examples thereof include apaper battery, a button battery, a coin battery, a layered-builtbattery, a cylindrical battery, and a rectangular battery. Besides alithium-ion secondary battery, the negative electrode material for alithium-ion secondary battery may be applied generally to anelectrochemical apparatus utilizing intercalation and de-intercalationof a lithium-ion as a charge-discharge mechanism, for example, a hybridcapacitor.

EXAMPLES

Hereinafter, the present invention is described more specifically byreferring to synthetic examples, Examples, and Comparative Examples.However, it should be noted that the invention is not limited to theseexamples.

Example 1

(1) First, 50 parts by mass of a coke powder having an average particlesize of 10 μm, 20 parts by mass of tar pitch, 20 parts by mass ofsilicon carbide, and 10 parts by mass of spherical natural graphitehaving an average particle size of 2θ μm (circularity: 0.92) were mixedtogether and stirred at 100° C. for 1 hour, thereby obtaining a mixture.Next, the obtained mixture was pulverized to a particle size of 25 μm,and the obtained pulverized powder was placed in a mold and formed intoa rectangular cuboid shape. The resultant was heat treated at 1000° C.in a nitrogen atmosphere, and then calcined at 2800° C. to graphitizethe graphitizable component. The molded graphite thus obtained waspulverized, thereby obtaining a graphite powder (a negative electrodematerial for a lithium-ion secondary battery).

With regard to the obtained graphite powder, the average particle size,the R-value, the pore volume, the specific surface area, the saturatedtap density, and the peak intensity ratio for the rhombohedral structurewere measured. The results are shown in Table 1. The respectivemeasurements were carried out according to the above-described methods.

(2) Then, 98 parts by mass of the graphite powder obtained above, 1 partby mass of a styrene-butadiene rubber (BM-400B, manufactured by NipponZeon Co., Ltd.), and 1 part by mass of a carboxymethyl cellulose (CMC2200, manufactured by Daicel Corporation) were mixed and kneaded,thereby obtaining a slurry. The obtained slurry was applied onto acurrent collector (a copper foil having a thickness of 10 μm), and theresultant was dried at 110° C. for 1 hour in an atmospheric air andintegrated using a roll press device under such conditions that theapplied material (active material) had an electrode density of 1.80g/cm³, thereby producing a negative electrode for a lithium-ionsecondary battery.

The orientation and peel strength of the negative electrode for alithium-ion secondary battery were measured according to the followingmethods. The measurement results are shown in Table 1.

<Orientation>

The orientation was determined using an X-ray diffraction measuringdevice with a CuKα ray as an X-ray source by measuring the surface of asample electrode. Specifically, the X-ray diffraction pattern of thesurface of the electrode sample was measured, and the orientation wasdetermined based on the intensity of the diffraction peak for (002)plane of carbon detected near the diffraction angle 2θ of from 26° to27° and the intensity of the diffraction peak for (110) plane of carbondetected near the diffraction angle 2θ of from 70° to 80°, in accordancewith the following Formula (1):Intensity of diffraction peak for (002) plane/Intensity of diffractionpeak for (110) plane  Formula (1)

<Peel Strength>

The measurement of the peel strength at the interface between thecurrent collector (copper foil) and the active material was conductedusing an autograph (manufactured by Shimadzu Corporation) by attachingan adhesive tape to the surface of the active material and pulling thetape vertically with respect to the electrode surface.

(3) A 2016-type coin cell was produced using the negative electrodeobtained above, metal lithium as a positive electrode, a mixed solutionof ethylene carbonate/ethyl methyl carbonate (volume ratio: 3/7) andvinylene carbonate (0.5% by mass) including 1.0 M LiPF₆ as anelectrolyte, a polyethylene microporous membrane having a thickness of25 μm as a separator, and a copper plate having a thickness of 230 μm asa spacer.

With respect to the lithium-ion secondary battery, each of the chargecapacity, the discharge capacity, the efficiency, the retention rate inrapid discharging, and the retention rate of low-temperature chargingwas measured by the following method. The measurement results are shownin Table 1.

<Charge Capacity and Discharge Capacity>

The charge-discharge capacity (first cycle charge-discharge capacity)was measured under the following conditions: the sample weight of 15.4mg, the electrode area of 1.54 cm², the measurement temperature of 25°C., the electrode density of 1700 kg/m³, the charge condition: constantcurrent charge of 0.434 mA, constant voltage charge of 0 V (Li/Li⁺), andcut current of 0.043 mA, and the discharge condition: constant currentcharge of 0.434 mA and cut voltage of 1.5 V (Li/Li⁺).

The discharge capacity was measured under the above charge condition anddischarge condition.

<Efficiency>

The efficiency was defined as a ratio (%) of the value of the measureddischarge capacity with respect to the value of the measured chargecapacity.

<Retention Rate in Rapid Discharging>

The retention rate in rapid discharging was measured in a thermostat at25° C. using the coin cell produced above according to the followingsteps (1) to (5).

(1) Charging was conducted by charging the cell at a constant current of0.434 mA up to 0 V (Vvs. Li/Li⁺), and then at a constant voltage of 0 Vuntil the current value reached 0.043 mA. The charging was paused for 30minutes, and then a charge capacity was measured.

(2) A first cycle test was conducted by discharging at a constantcurrent of 0.434 mA to 1.5 V (Vvs. Li/Li⁺) and pausing for 30 minutes,and then the discharge capacity was measured.

(3) At the second cycle, the charging and the discharging of the steps(1) and (2) were repeated, and the charge capacity and the dischargecapacity were measured.

(4) At the third cycle and thereafter, the measurement was performedunder the same charging condition as in the step (1) and under adischarge condition that the constant current value of the step (2) waschanged to 4.34 mA (third cycle), 6.51 mA (fourth cycle), 8.68 mA (fifthcycle) 10.85 mA (sixth cycle), or 13.02 mA (2.4 C) (seventh cycle).

(5) With regard to the measurement of the retention rate in rapiddischarging, the retention rate (%) was calculated by dividingrespective discharge capacities measured at the third to the seventhcycles by the discharge capacity at the second cycle.

<Retention Rate in Low-temperature Charging>

The retention rate in low-temperature charging was measured using thecoin cell produced above according to the following steps (6) to (8).

(6) Charging and discharging were conducted in a thermostat at 25° C.according to the above steps (1), (2), and (3), and then charge capacitywas measured.

(7) After the pausing of discharging of the step (6) was finished andthe temperature inside the thermostat reached 0° C., charging wasconducted as in the step (1) while maintaining the temperature at 0° C.,and then the charge capacity was measured.

(8) With regard to the measurement of the retention rate inlow-temperature charging, the retention rate (%) was calculated bydividing the charge capacity at a time when the voltage at the constantcurrent of 0.434 mA reached 0 V (Vvs. Li/Li⁺) at 0° C. in the step (6)by the charge capacity at a time when the voltage at the constantcurrent of 0.434 mA reached 0 V (Vvs. Li/Li⁺) at 25° C. in the step (3).

Example 2

The graphite powder obtained in Example 1 was placed in a rubbercontainer, and the rubber container was sealed. The rubber container wasthen subjected to an isotropic pressing treatment using a pressingmachine at a pressure of 9800 N/cm² (1000 kgf/cm²) The graphite powderwas then crushed using a cutter mill and graded with a sieve, therebyobtaining a graphite powder of Example 2.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 3

A graphite powder of Example 3 was obtained in the same manner as inExample 2 except that the coke powder, the tar pitch, and the siliconcarbide were mixed, stirred, and pulverized to obtain a pulverizedpowder, and then the spherical natural graphite was mixed with theobtained pulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 4

A graphite powder of Example 4 was obtained in the same manner as inExample 2 except that the coke powder, the tar pitch, and the sphericalnatural graphite were mixed, stirred, and pulverized to obtain apulverized powder, and then the silicon carbide was mixed with thepulverized powder.

A negative electrode for lithium-ion secondary battery and a lithium-ionsecondary battery were produced in the same manner as in Example 1, andmeasurements were performed in the same manner as in Example 1. Theresults are shown in Table 1.

Example 5

A graphite powder of Example 5 was obtained in the same manner as inExample 2 except that the coke powder and the tar pitch were mixed,stirred, and pulverized to obtain a pulverized powder, and then thesilicon carbide and the spherical natural graphite were mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 6

A graphite powder of Example 6 was obtained in the same manner as inExample 2 except that the amounts of the coke powder, the tar pitch, thesilicon carbide, and the spherical natural graphite were changed to 43parts by mass, 18.5 parts by mass, 18.5 parts by mass, and 20 parts bymass, respectively.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 7

A graphite powder of Example 7 was obtained in the same manner as inExample 6 except that the coke powder, the tar pitch, and the siliconcarbide were mixed, stirred, and pulverized to obtain a pulverizedpowder, and then the spherical natural graphite was mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 8

A graphite powder of Example 8 was obtained in the same manner as inExample 6 except that the coke powder, the tar pitch, and the sphericalnatural graphite were mixed, stirred, and pulverized to obtain apulverized powder, and then the silicon carbide was mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 9

A graphite powder of Example 9 was obtained in the same manner as inExample 6 except that the coke powder and the tar pitch were mixed,stirred, and pulverized to obtain a pulverized powder, and then thesilicon carbide and the spherical natural graphite were mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 10

A graphite powder of Example 10 was obtained in the same manner as inExample 2 except that the amounts of the coke powder, the tar pitch, thesilicon carbide, and the spherical natural graphite were changed to 41parts by mass, 16 parts by mass, 16 parts by mass, and 27 parts by mass,respectively.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 11

A graphite powder of Example 11 was obtained in the same manner as inExample 10 except that the coke powder, the tar pitch, and the siliconcarbide were mixed, stirred, and pulverized to obtain a pulverizedpowder, and then the spherical natural graphite was mixed with thepulverized powder.

A negative electrode for lithium-ion secondary battery and a lithium-ionsecondary battery were produced in the same manner as in Example 1, andmeasurements were performed in the same manner as in Example 1. Theresults are shown in Table 1.

Example 12

A graphite powder of Example 12 was obtained in the same manner as inExample 10 except that the coke powder, the tar pitch, and the sphericalnatural graphite were mixed, stirred, and pulverized to obtain apulverized powder, and then the silicon carbide was mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 13

A graphite powder of Example 13 was obtained in the same manner as inExample 10 except that the coke powder and the tar pitch were mixed,stirred, and pulverized to obtain a pulverized powder, and then thesilicon carbide and the spherical natural graphite were mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 14

A graphite powder of Example 14 was obtained in the same manner as inExample 2 except that the amounts of the coke powder, the tar pitch, thesilicon carbide, and the spherical natural graphite were changed to 29parts by mass, 11 parts by mass, 5 parts by mass, and 55 parts by mass,respectively.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 15

A graphite powder of Example 15 was obtained in the same manner as inExample 14 except that the coke powder, the tar pitch, and the siliconcarbide were mixed, stirred, and pulverized to obtain a pulverizedpowder, and then the spherical natural graphite was mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 16

A graphite powder of Example 16 was obtained in the same manner as inExample 14 except that the coke powder, the tar pitch, and the sphericalnatural graphite were mixed, stirred, and pulverized to obtain apulverized powder, and then the silicon carbide was mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 17

A graphite powder of Example 17 was obtained in the same manner as inExample 14 except that the coke powder and the tar pitch were mixed,stirred, and pulverized to obtain a pulverized powder, and then thesilicon carbide and the spherical natural graphite were mixed with thepulverized powder.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 18

A graphite powder of Example 18 was obtained in the same manner as inExample 9 except that, instead of using the coke powder used in Example9, the same amount of a mosaic coke having a lower crystallinity thanthe coke powder was used.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Comparative Example 1

First, 100 parts by mass of a coke powder, 40 parts by mass of tarpitch, and 25 parts by mass of silicon carbide were mixed and heated at250° C. The obtained mixture was pulverized and then pressure-moldedinto a pellet form. The obtained pellet was calcined at 900° C. innitrogen and then graphitized at 3000° C. in a graphitization furnace.The obtained graphitized product was pulverized using a hammer mill andgraded with a sieve, thereby obtaining a graphite powder having anaverage particle size of 21 μm.

Example 19

A graphite powder of Example 19 was obtained in the same manner as inExample 9 except that, instead of using the spherical natural graphiteused in Example 9, the same amount of spherical artificial graphitehaving an average particle size of 22 μm (circularity: 0.78) was used.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 20

A graphite powder of Example 2θ was obtained in the same manner as inExample 9 except that, instead of using the spherical natural graphiteused in Example 9, the same amount of spherical natural graphite havingan average particle size of 23 μm (circularity: 0.95) was used.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Example 21

A graphite powder of Example 21 was obtained in the same manner as inExample 9 except that, instead of using the spherical natural graphiteused in Example 9, the same amount of spherical natural graphite havingan average particle size of 10 μm (circularity: 0.90) was used.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Comparative Example 2

A graphite powder of Comparative Example 2 was obtained in the samemanner as in Example 9 except that, instead of using the sphericalnatural graphite described in Example 9, the same amount of flake-shapednatural graphite having an average particle size of 25 μm was used.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Comparative Example 3

A graphite powder of Comparative Example 3 was obtained in the samemanner as in Example 9 except that, instead of using the sphericalnatural graphite described in Example 9, the same amount of flake-shapednatural graphite that had been graded to a size of 2θ μm using a sievewas used.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Comparative Example 4

Only spherical natural graphite as used in Comparative Example 1 wasplaced in a graphite crucible and calcined at 2800° C. in a nitrogenatmosphere, thereby obtaining a graphite powder of Comparative Example4.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

Comparative Example 5

A graphite powder of Comparative Example 5 was obtained in the samemanner as in Example 1 except that the coke powder as in Example 1 wasnot used.

A negative electrode for a lithium-ion secondary battery and alithium-ion secondary battery were produced in the same manner as inExample 1, and measurements were performed in the same manner as inExample 1. The results are shown in Table 1.

TABLE 1 Retention Retention rate in rate in Peak intensity rapid low-Average Specific Saturated ratio for Dis- dis- temper- particle R- Poresurface tap rhombohedral Orien- Peel Charge charge Effi- charging aturesize value volume area density structure tation strength capacitycapacity ciency (2.4 C) charging Unit Item μm — g/cm³ m²/g m²/g — — mNmAh/g mAh/g % % % Example 1 24.0 0.04 0.76 2.6 0.80 0.12 388 25 382 36094.3 80 52 Example 2 23.4 0.07 0.62 3.5 0.96 0.17 417 30 384 362 94.4 8252 Example 3 23.1 0.05 0.62 3.6 0.94 0.19 417 30 384 362 94.4 82 51Example 4 23.2 0.07 0.61 3.3 0.96 0.14 429 30 383 362 94.5 80 50 Example5 23.3 0.06 0.61 3.4 0.93 0.18 418 30 382 361 94.5 80 50 Example 6 23.10.06 0.60 3.6 0.96 0.21 412 32 383 361 94.3 83 53 Example 7 22.8 0.070.60 3.6 0.96 0.19 422 32 383 362 94.4 83 56 Example 8 23.0 0.07 0.593.5 0.97 0.25 409 32 384 362 94.4 82 52 Example 9 23.0 0.07 0.59 3.60.95 0.20 410 32 383 362 94.4 82 52 Example 10 22.9 0.07 0.55 3.7 0.980.27 407 33 383 361 94.2 82 54 Example 11 22.7 0.08 0.54 3.8 0.99 0.23420 33 384 362 94.3 82 54 Example 12 22.8 0.07 0.56 3.7 0.99 0.16 411 33385 363 94.3 83 52 Example 13 22.7 0.09 0.57 3.8 1.00 0.23 402 33 382360 94.2 83 52 Example 14 22.4 0.08 0.53 4.2 1.04 0.19 488 34 382 35994.0 79 54 Example 15 22.5 0.08 0.53 4.3 1.05 0.23 501 34 382 359 94.180 54 Example 16 22.5 0.07 0.51 4.2 1.04 0.21 498 34 383 360 94.0 78 53Example 17 22.6 0.10 0.52 4.2 1.04 0.30 503 34 381 358 93.9 77 51Example 18 23.2 0.09 0.61 3.8 0.93 0.26 315 32 381 360 94.7 85 59Example 19 22.8 0.06 0.60 3.0 0.88 0.21 419 28 383 362 94.6 85 51Example 20 23.5 0.07 0.56 3.2 0.99 0.19 419 33 382 361 94.5 83 54Example 21 20.5 0.12 0.62 4.1 0.90 0.26 515 26 384 360 93.7 78 62Comparative 21.0 0.07 0.64 4.2 0.79 0.20 554 25 372 357 93.5 73 49Example 1 Comparative 22.4 0.06 0.58 3.9 0.81 0.18 838 24 383 355 92.770 50 Example 2 Comparative 20.3 0.25 0.60 5.5 0.94 0.36 618 68 386 35692.2 63 63 Example 3 Comparative 20.3 0.03 0.60 4.1 0.94 0.12 671 44 382358 93.7 62 50 Example 4 Comparative 21.0 0.05 0.50 2.8 1.16 0.31 537 61382 360 94.2 66 53 Example 5

Each of the graphite powders of Examples 1 to 21 included a compositeparticle including a spherical graphite particle and plural graphiteparticles that have a compressed shape and aggregate or are combined soas to have nonparallel orientation planes.

In addition, as shown in Table 1, the negative electrode material for alithium-ion secondary battery produced in each of the Examples exhibitedan improved retention rate in rapid discharging (load characteristics)as compared to the negative electrode material for a lithium-ionsecondary battery produced in each of the Comparative Examples.

The disclosure of Japanese Patent Application No. 2014-062431 isentirely incorporated herein by reference. All documents, patentapplications, and technical standards described in the presentspecification are incorporated herein by reference to the same extent asif each individual document, patent application, and technical standardwere specifically and individually indicated to be incorporated byreference.

The invention claimed is:
 1. A negative electrode material for alithium-ion secondary battery, the negative electrode materialcomprising a composite particle including a spherical graphite particleand a plurality of graphite particles that have a compressed shape andthat aggregate or are combined so as to have nonparallel orientationplanes, and the negative electrode material having an R-value in a Ramanmeasurement of from 0.03 to 0.10, and having a pore volume as obtainedby mercury porosimetry of from 0.2 mL/g to 1.0 mL/g in a pore diameterrange of from 0.1 μm to 8 μm.
 2. The negative electrode material for alithium-ion secondary battery according to claim 1, wherein a specificsurface area of the negative electrode material, as measured by a BETmethod, is from 1.5 m²/g to 6.0 m²/g.
 3. The negative electrode materialfor a lithium-ion secondary battery according to claim 1, wherein asaturated tap density of the negative electrode material is from 0.8 to1.2 g/cm³.
 4. The negative electrode material for a lithium-ionsecondary battery according to claim 1, wherein of the negativeelectrode material has an intensity ratio (P2/P1) of a diffraction peak(P2) for a (101) plane of a rhombohedral crystal structure to adiffraction peak (P1) of a (101) plane for a hexagonal crystal structurein a CuKα X-ray diffraction pattern is 0.35 or less.
 5. The negativeelectrode material for a lithium-ion secondary battery according toclaim 1, wherein the spherical graphite particle has a circularity of0.8 or higher.
 6. A method of manufacturing the negative electrodematerial for a lithium-ion secondary battery according to claim 1, themethod comprising steps of: (a) mixing a graphitizable aggregate orgraphite with a graphitizable binder, a graphitization catalyst, and aspherical graphite particle; and (b) calcining the mixture.
 7. Themethod of manufacturing the negative electrode material for alithium-ion secondary battery according to claim 6, comprising, betweenthe steps (a) and (b), at least one step selected from the groupconsisting of (c) molding the mixture and (d) subjecting the mixture toa heat treatment.
 8. A negative electrode material slurry a lithium-ionsecondary battery, the negative electrode material slurry comprising:the negative electrode material for a lithium-ion secondary batteryaccording to claim 1; an organic binder; and a solvent.
 9. A negativeelectrode for a lithium-ion secondary battery, the negative electrodecomprising: a current collector; and a negative electrode material layerformed on the current collector and comprising the negative electrodematerial for a lithium-ion secondary battery according to claim
 1. 10. Alithium-ion secondary battery, comprising: a positive electrode; anelectrolyte; and the negative electrode for a lithium-ion secondarybattery according to claim 9.